Tire size calculator
Compare two tire sizes: diameter, sidewall height and speedometer error.
About this calculator
This tire size calculator compares your current tires with a new size. Enter both sizes as written on the sidewall, such as 225/45R17.
It shows the difference in overall diameter, sidewall height and circumference, and how much your speedometer will be off.
How it works
In 225/45R17, 225 is the tread width in millimetres, 45 means the sidewall is 45% of that width, and 17 is the wheel diameter in inches. The overall diameter is the wheel diameter plus twice the sidewall height.
Last reviewed:
Worked example
- Current tire size: 225/45R17
- New tire size: 235/40R18
Diameter difference: +1.72% (within the usual 3% guideline)
Common questions
What does 225/45R17 mean?
A tread width of 225 mm, a sidewall 45% of that width (101.25 mm), radial construction and a 17-inch wheel.
How much can the tire diameter change?
Most fitters keep the overall diameter within about 3% of the original, so the speedometer, ABS and gearing still work as designed.
Do bigger tires make my speedometer wrong?
Yes. A taller tire travels further on each turn, so you go faster than the speedometer shows. A smaller one makes it read high.
Sources and further reading
The story behind it
Where the idea came from: Metric tire sizes such as 225/45R17 became standard in the 1970s, as radial tires replaced the older bias-ply type.
Fitting a different size changes how far the car travels on each turn of the wheel, so the speedometer and odometer read differently. Keeping the overall diameter within about 3% is the usual rule.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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